1. Field of the Invention
Embodiments of the invention generally relate to a sensing unit that processes electrical signals of a human or animal body and specifically enables earlier signal detection after stimulation than known solutions that cannot identify natural activities of the body tissue for a specific period of time after stimulation, e.g., during digital blanking.
2. Description of the Related Art
Sensing units are generally a component of tissue stimulators such as cardiac pacemakers, defibrillators, cardioverters, neurostimulators and the like and are used for the processing of electrical signals of the body tissue, which are normally captured by means of electrodes at, on, or in the tissue. A typical sensing unit is connected via an electrode connection to such an electrode and includes at least one analogue-digital converter (A/D converter) and a digital filter stage. The further processing of the output signals of the sensing unit comprises at least an assessment of the signal amplitudes, for example electrical signals of the body tissue with a amplitude above a specific threshold value are thus identified as natural activity and are assigned to specific physiological events on the basis of their characteristics. For stimulation of the body tissue, the tissue stimulator contains a stimulation unit, via which one or more electric pulses are delivered to the body tissue via the electrode. Since the amplitude of the stimulation pulses is usually a multiple of the amplitudes of the natural electric tissue activity, the connection between the sensing unit and the electrode is interrupted at least for the duration of the stimulation (for example, with a stimulation pulse duration of 1 ms, this time of separation of the sensing unit from the electrode may be 8 to 20 ms).
Generally, stimulation pulses may cause polarization effects (charge reversal effects at the boundary layer between the electrode surface and the electrolyte) at the electrode in the electrolyte or the surrounding tissue and may lead to a step of the amplitude of the input signal of the sensing unit when connected to the electrode after the stimulation. In particular, filters of a sensing unit typically respond to this step in amplitude of the input signal by a step response, which manifests itself as a longer lasting increased amplitude of the output signal of the sensing unit and is also generally referred to as a stimulation artifact. Such stimulation artifacts, generally, may negatively influence the function of the tissue stimulator because a stimulation artifact may be misinterpreted incorrectly as natural electric tissue activity.
Typically, the output signals of the sensing unit are therefore excluded from the signal detection for a specific period of time after the stimulation, also referred to as digital blanking. A disadvantage of excluding the output signals from the signal detection, generally, is that actual natural activities of the body tissue cannot be identified during the period of digital blanking.
As such, in view of the above, there is a need for a sensing unit that overcomes this disadvantage and that enables earlier signal detection after stimulation.
For analog/digital (A/D) conversion, time-continuous analogue signals are normally sampled in the sensing unit at a specific sampling frequency and are converted into digitally coded time-discrete and amplitude-discrete sampling values. The sampling frequency is fed to the A/D converter as a clock frequency. The A/D converter provides a new sampled value with each clock step of the clock frequency. With the subsequent digital filtering, the sampled values are subjected to mathematical operations, such as multiplication and/or addition with filter coefficients in accordance with the selected filter algorithm, wherein the same clock frequency as with the A/D conversion is used. The digital filter thus delivers a modified sampled value at the output with each clock step for the sampled value present at the input.